Process and apparatus for the production of para-xylene

ABSTRACT

A process for producing para-xylene (PX) comprises supplying a hydrocarbon feed comprising xylenes and ethylbenzene (EB) to a PX recovery unit, where a PX-rich stream and at least one PX-depleted stream are recovered from the feed. The PX-depleted stream is then separated into an EB-rich stream and an EB-depleted stream in a divided wall column. The EB-depleted stream is then isomerized under at least partial liquid phase conditions to produce a first isomerized stream having a higher PX concentration than the PX-depleted stream, and the EB-rich stream is isomerized under at least partial vapor phase conditions to produce a second isomerized stream having a higher PX concentration than the PX-depleted stream. The first and second isomerized streams are then recycled to the PX recovery unit to recover additional PX and the process is repeated to define a so-called xylene isomerization loop.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/969,298, now U.S. Pat. No. 9, 517, 979 filed Dec. 15, 2015, whichclaims the benefit of Provisional Application No. 62/135,255, filed Mar.19, 2015, the disclosures of which are incorporated herein by referencein their entireties.

FIELD

This application relates to the treatment of ethylbenzene in theproduction of para-xylene.

BACKGROUND

Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene(MX) are often present together in C₈ aromatic product streams fromchemical plants and oil refineries. Of these C₈ compounds, although highpurity EB is an important raw material for the production of styrene,for a variety of reasons all high purity EB feedstocks used in styreneproduction are produced by alkylation of benzene with ethylene, ratherthan by recovery from a C₈ aromatics stream. Of the three xyleneisomers, PX has the largest commercial market and is used primarily formanufacturing terephthalic acid and terephthalate esters for use in theproduction of various polymers such as poly(ethylene terephthalate),poly(propylene terephthalate), and poly(butene terephthalate). While OXand MX are useful as solvents and raw materials for making products suchas phthalic anhydride and isophthalic acid, market demand for OX and MXand their downstream derivatives is much smaller than that for PX.

Given the higher demand for PX as compared with its other isomers, thereis significant commercial interest in maximizing PX production from anygiven source of C₈ aromatic materials. However, there are a number ofmajor technical challenges to be overcome in achieving this goal ofmaximizing PX yield. For example, the four C₈ aromatic compounds,particularly the three xylene isomers, are usually present inconcentrations dictated by the thermodynamics of production of the C₈aromatic stream in a particular plant or refinery. As a result, the PXproduction is limited, at most, to the amount originally present in theC₈ aromatic stream unless additional processing steps are used toincrease the amount of PX and/or to improve the PX recovery efficiency.A variety of methods are known to increase the concentration of PX in aC₈ aromatics stream. These methods normally involve cycling the streambetween a separation step, in which at least part of the PX is recoveredto produce a PX-depleted stream, and a xylene isomerization step, inwhich the PX content of the PX-depleted stream is returned back towardsequilibrium concentration.

In a typical aromatics plant, such as that shown in FIG. 1, liquid feed,typically a C₈₊ aromatic feedstream which has previously been processedby known methods to remove C⁷⁻ species (particularly benzene andtoluene), is fed by conduit 1 to xylenes re-run 3, an apparatus per sewell known in the art. The xylenes re-run (or more simply afractionation column) vaporizes the feed and separates the C₈ aromaticsinto an overhead mixture 5 of xylenes (OX, MX, and PX) and ethylbenzene(EB), and a bottom product 61 comprising C₉₊ aromatics. The overheadmixture typically has a composition of about 40-50% metaxylene (MX),15-25% PX, 15-25% OX, and 10-20% EB. Unless otherwise noted herein,percentages are % weight. The overhead is then condensed in condenser 7,an apparatus also per se well-known in the art, and becomes the feed forthe PX recovery unit 15, via conduit 9 and 13, a portion of thecondensed overhead may be returned to re-run 3 as reflux via conduits 9and 11.

The PX recovery unit 15 may employ crystallization technology,adsorption technology, or membrane separation technology, each per sewell known in the art. These technologies separate PX from its isomersand are capable of producing high purity PX up to 99.9%, which is takenfrom unit 15 via conduit 17. Shown in FIG. 1 is the case where unit 15is an adsorptive separation unit, such as a Parex™ or Ehuxyl™ unit, inwhich case typically the extract 17, which comprises a desorbent, suchas paradiethylbenzene (PDEB), needs to be separated, such as bydistillation, from the desired extract PX in distillation column 19,which generates an overhead 23 that is condensed in condenser 25 toyield a liquid stream 27, which is a high purity PX stream. This stream27 may be taken off via conduit 31 and optionally a portion may bereturned to column 19 as reflux via conduit 29. The desorbent isreturned to the PX recovery system 15 via conduit 21. Raffinate from therecovery system 15, comprising MX, OX, EB, and some PX, is removed viaconduit 65 and sent to unit 37, discussed below. Note: a portion ofraffinate in 65 may be recovered and marketed as low-value solventxylene.

The raffinate 65, which comprises mainly MX, OX, EB, and desorbent issent to fractionation column 37, generating overhead 33 and bottoms 63.Overhead 33 contains MX, OX and EB, which is condensed in condenser 32and sent via conduit 35 and then 41 to isomerization unit 43, discussedin more detail below. A portion may be returned to fractionator 37 viaconduit 35 and then 39 as reflux. The desorbent in the bottoms productis returned to 15. Note that as used herein the term “raffinate” is usedto mean the portion recovered from the PX recovery unit 15, whether thetechnology used is adsorptive separation, crystallization, or membrane,and then is sent to the isomerization unit 43, conventionally a vaporphase isomerization unit, which uses technology also per se well-known.

A stream consisting essentially of MX, OX and EB is sent toisomerization unit 43, an apparatus per se known in the art, toisomerize the MX and OX and optionally EB to PX. Isomerization unit 43may be a vapor phase or liquid phase isomerization unit. Conventionallythere are one or more heat exchangers or furnaces associated with thesystem shown in FIG. 1 between the PX recovery unit 15 and theisomerization unit that are not shown for convenience of view. Likewise,hydrogen separators and hydrogen compressors are also not shown forconvenience of view. These and other features, such as valves and thelike, would be apparent to one of ordinary skill in the art inpossession of the present invention.

The product of the isomerization unit 43 is sent via conduit 51 to theC⁷⁻ distillation tower 53, which separates the product of isomerizationinto a bottom stream 59 comprising equilibrium xylenes and the overhead47, comprising C⁷⁻ aromatics, e.g., benzene and toluene. The overheadproduct is condensed in condenser 45 and then the distribution of liquidproduct via conduit 49 may be apportioned as desired between conduit 57and conduit 55, the former of which may be disposed of in numerous wayswhich would be well-known per se in the art, and the latter conduitreturning C⁷⁻ aromatics as reflux to tower 53. The bottoms product 59 ofdistillation tower 53 is then sent to xylenes re-run 3, either mergingwith feed 1 as shown in the figure, or it may be introduced by aseparate inlet (not shown).

The xylene isomerization unit 43 may be conducted in either a vaporphase or liquid phase and is intended to accomplish two major things:isomerize the lower valued MX and OX to higher value PX, and convert EBinto benzene/toluene and light gases (so-called “EB destruction”) oroptionally, isomerize EB to xylenes. Various options exist for using oneor more of the xylenes isomerization technologies, but generally,conducting the xylene isomerization under at least partially liquidphase conditions minimizes xylene loss and is more energy efficient thanvapor phase isomerization. However, under these conditions, little ornone of the EB may be converted in the xylene isomerization step and asa result the amount of EB in the xylenes loop can build up to very highlevels. Thus, to maximize the use of liquid phase isomerization, it isdesirable to control the amount of EB in the PX-depleted streamsubjected to liquid phase isomerization.

SUMMARY

The present invention is directed to a process for producing PX in whicha hydrocarbon feed comprising xylenes and EB is provided to a PXrecovery unit, which recovers a PX-rich stream and at least onePX-depleted stream. The at least one PX-depleted stream is sent to adivided wall column where it is separated into an EB-rich stream and anEB-depleted stream. At least a portion of the EB-depleted stream isisomerized at least partially in the liquid phase to produce a firstisomerized stream having a higher PX concentration than the PX-depletedstream, and at least a portion of the EB-rich stream is isomerized atleast partially in the vapor phase to produce a second isomerized streamhaving a higher PX concentration than the PX-depleted stream. The firstand second isomerized streams are then recycled to the PX recovery unit.In one embodiment, the PX recovery unit produces a single PX-depletedstream, while in another embodiment, the PX recovery unit produces twoPX-depleted streams—one rich in EB and one low in EB.

The invention further provides an apparatus for the production of PXcomprising a PX recovery unit, which produces a PX-rich stream and atleast one PX-depleted stream from a hydrocarbon feed, fluidly connectedto a divided wall column in which the at least one PX-depleted stream isseparated into an EB-rich stream and an EB-depleted stream; a liquidphase isomerization unit fluidly connected to the divided wall column toisomerize the EB-depleted stream and produce a first isomerized streamhaving a higher PX concentration than the PX-depleted stream; and avapor phase isomerization unit fluidly connected to the divided wallcolumn to isomerize the EB-rich stream and produce a second isomerizedstream having a higher PX concentration than the PX-depleted stream. Theapparatus may also include a xylenes fractionation column fluidlyconnected to the liquid phase isomerization unit, the vapor phaseisomerization unit and the PX recovery unit, downstream of the liquidphase isomerization unit and the vapor phase isomerization unit andupstream of the PX recovery unit, and a fractionation column fluidlyconnected to the vapor phase isomerization unit and the xylenesfractionation column, downstream of the vapor phase isomerization unitand upstream of the xylenes fractionation column.

The present invention provides a process for controlling the amount ofEB in the PX-depleted stream and maximizing the use of liquid phaseisomerization, which minimizes xylene loss and results in energysavings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a conventional para-xylene (PX) productionand extraction process which employs liquid phase xylene isomerizationand vapor phase xylene isomerization.

FIG. 2 is a flow diagram of one embodiment of the inventive process.

FIG. 3 is a flow diagram of a second embodiment of the inventiveprocess.

DETAILED DESCRIPTION OF THE DRAWINGS

As used herein the term “C_(n)” hydrocarbon, wherein n is a positiveinteger, means a hydrocarbon having n number of carbon atom(s) permolecule. For example, a C₈ aromatic hydrocarbon means an aromatichydrocarbon or mixture of aromatic hydrocarbons having 8 number ofcarbon atom(s) per molecule. The term “C_(n)+” hydrocarbon, wherein n isa positive integer, means a hydrocarbon having at least n number ofcarbon atom(s) per molecule, whereas the term “C_(n)−” hydrocarbonwherein n is a positive integer, means a hydrocarbon having no more thann number of carbon atom(s) per molecule.

The present invention is an improved process for producing PX. Withreference to FIG. 2, a hydrocarbon feed 102 comprising xylenes and EB isprovided to a PX recovery unit 120, where a PX-rich stream 122 and atleast one PX-depleted stream 124 are recovered from the feed. ThePX-depleted stream 124 is then separated into an EB-rich stream 132 andan EB-depleted stream 134 in a divided wall column 130. At least part ofthe EB-depleted stream 134 is then fed to a xylene isomerization unit150 where the EB-depleted stream 134 is isomerized under at leastpartial liquid phase conditions to produce a first isomerized stream 152having a higher PX concentration than the PX-depleted stream 124. Atleast part of the EB-rich stream 132 is then fed to a xyleneisomerization unit 140 where the EB-rich stream 132 is isomerized underat least partial vapor phase conditions to produce a second isomerizedstream 142 having a higher PX concentration than the PX-depleted stream124. At least part of the first and second isomerized streams 152, 142are then recycled to the PX recovery unit 120 to recover additional PXand the process is repeated to define a so-called xylene isomerizationloop.

Hydrocarbon Feed

The hydrocarbon feed 102 employed in the present process may be anyhydrocarbon stream containing C₈ aromatic hydrocarbons, such as areformate stream (product stream of a reformate splitting tower), ahydrocracking product stream, a xylene or EB reaction product stream, anaromatic alkylation product stream, an aromatic disproportionationstream, an aromatic transalkylation stream, a methanol to aromaticproduct stream, and/or a Cyclar™ process stream.

In one embodiment, the feed is the product of the alkylation of benzeneand/or toluene with methanol and/or dimethyl ether in a methylationreactor. One such methylation reactor is described in U.S. Pat. Nos.6,423,879 and 6,504,072, the entire contents of which are incorporatedherein by reference, and employs a catalyst comprising a porouscrystalline material having a Diffusion Parameter for 2,2 dimethylbutaneof about 0.1-15 sec⁻¹ when measured at a temperature of 120° C. and a2,2 dimethylbutane pressure of 60 torr (8 kPa). The porous crystallinematerial may be a medium-pore zeolite, such as ZSM-5, which has beenseverely steamed at a temperature of at least 950° C. in the presence ofat least one oxide modifier, for example including phosphorus, tocontrol reduction of the micropore volume of the material during thesteaming step. Such a methylation reactor is hereinafter termed a“PX-selective methylation reactor”.

The feedstock may further comprise recycle stream(s) from theisomerization step(s) and/or various separating steps. The hydrocarbonfeed comprises PX, together with meta-xylene (MX), ortho-xylene (OX),and ethylbenzene (EB). In addition to xylenes and EB, the hydrocarbonfeedstock may also contain certain amounts of other aromatic or evennon-aromatic compounds. Examples of such aromatic compounds are C₇-hydrocarbons, such as benzene and toluene, and C₉+ aromatics, such asmesitylene, pseudo-cumene and others. These types of feedstream(s) aredescribed in “Handbook of Petroleum Refining Processes”, Eds. Robert A.Meyers, McGraw-Hill Book Company, Second Edition.

Para-Xylene Recovery

The hydrocarbon feed 102 is initially supplied to a PX recovery unit 120to recover a PX-rich product stream 122 from the feed and leave aPX-depleted stream 124. In one embodiment, the PX-rich product stream122 comprises at least 50 wt % PX, preferably at least 60 wt % PX, morepreferably at least 70 wt % PX, even more preferably at least 80 wt %PX, most preferably at least 90 wt % PX, and ideally at least 95 wt %PX, based on the total weight of the PX rich product stream. The PXrecovery unit 120 can include one or more of any of the PX recoveryunits known in the art, including, for example, a crystallization unit,an adsorption unit such as a PAREX™ unit or an ELUXYL™ unit, a reactiveseparation unit, a membrane separation unit, an extraction unit, adistillation unit, an extractive distillation unit, a fractionationunit, or any combination thereof These types of separation unit(s) andtheir designs are described in “Perry's Chemical Engineers' Handbook”,Eds. R. H. Perry, D. W. Green and J. O. Maloney, McGraw-Hill BookCompany, Sixth Edition, 1984, and the previously-mentioned “Handbook ofPetroleum Refining Processes”. In a preferred embodiment, the PXrecovery unit 120 is a simulated moving bed adsorption unit such as aPAREX™ unit or an ELUXYL™ unit.

Depending on the composition of the hydrocarbon feed 102, one or moreinitial separation steps that serve to remove C₇− and C₉+ hydrocarbonsfrom the feed may occur prior to recovery of the PX-rich product stream122. Generally the initial separation steps may include fractionaldistillation, crystallization, adsorption, a reactive separation, amembrane separation, extraction, or any combination thereof In oneembodiment, the feed 102 is passed through a xylenes fractionation tower110 prior to passing to the PX recovery unit 120. The xylenesfractionation tower 110 produces an overhead stream 112 comprising C₈hydrocarbons and a bottoms stream 114 containing C₉₊ hydrocarbons.

In the embodiment shown in FIG. 2, the PX recovery unit 120 produces theconventional raffinate, or PX-depleted stream 124, comprising MX, OX andEB. In another embodiment, shown in FIG. 3, the PX recovery unit 120 maybe modified to produce two different PX-depleted streams—a PX-depletedstream 126 rich in EB and a PX-depleted stream 128 low in EB. Examplesof PX recovery units that produce multiple raffinate streams aredescribed in U.S. patent application Ser. No. 14/624,861 and U.S. Pat.Nos. 7,582,206 and 8,030,533.

Divided Wall Column

Returning to FIG. 2, the PX-depleted stream 124 then passes to a dividedwall column 130, which separates the PX-depleted stream 124 into twostreams—an EB-rich stream 132 and an EB-depleted stream 134. The dividedwall column 130 may be used in addition to, or, in a preferredembodiment, in place of, the raffinate tower (fractionator 37 in FIG.1). As its name implies, the term “divided wall distillation column”refers to a particular known form of distillation column which comprisesa dividing wall. The dividing wall vertically bisects a portion of theinterior of the distillation column but does not extend either to thetop or bottom sections of the column, thus, enabling the column to berefluxed and reboiled similar to a conventional column. The dividingwall provides a fluid impermeable baffle separating the interior of thecolumn. The divided wall column may be configured for a number ofprocesses, with an inlet to the column is located on one side of thedividing wall and one or more side draws are located on the opposingside, or an inlet on both sides of the divided wall column and multipledraws from the top or bottom of the column, or any combination thereof.

In a particular embodiment, the dividing wall extends from the top ofthe column down to a tray on which the EB concentration is low enough toprovide an optimum ratio of low EB and high EB products, which isascertainable by one skilled in the art using simulation tools. ThePX-depleted stream 124 is provided to the column on one side of thedividing wall and the two streams are withdrawn from the top of thecolumn. Each overhead stream is processed through a separate overheadproduct system 136, 138, each consisting of a condenser, which may bedrumless, reflux pump, and an optional reflux drum. A portion of theEB-rich stream 132 may be returned to the divided wall column 130 asreflux, while the remainder of the EB-rich stream 132 is sent toisomerization unit 140. A portion of the EB-depleted stream 134 may bereturned to the divided wall column 130 as reflux, while the remainderof the EB-depleted stream 134 is sent to isomerization unit 150. Inother embodiments in which the amount of EB in the EB-rich stream isminimal, the EB-rich stream may be purged to fuel blending.

In the embodiment depicted in FIG. 3, the PX-depleted stream 126 rich inEB enters the divided wall column 130 on one side of the dividing walland the PX-depleted stream 128 low in EB enters the divided wall column130 on the opposite side of the dividing wall. The divided wall column130 further enhances the EB separation, and an EB-rich stream 132 and anEB-depleted stream 134 are recovered as overhead streams as in theprevious embodiment. Using a divided wall column in conjunction with asimulated moving bed adsorption unit modified to separate the raffinateinto high and low EB streams not only eliminates the need for theconventional raffinate tower, but the divided wall column providesadditional EB separation beyond what the simulated moving bed adsorptionunit achieves.

Xylene Isomerization

Because liquid phase isomerization converts little or none of the EB inthe PX-depleted stream, in a preferred embodiment, the EB-depletedstream 134 is sent to isomerization unit 150, which is operated in theliquid phase, and the EB-rich stream 132 is sent to isomerization unit140, which is operated in the vapor phase. Minimizing the amount ofPX-depleted stream subjected to vapor phase isomerization saves energyand capital, as liquid phase isomerization requires less energy andcapital than the vapor phase isomerization process due to therequirement of vaporizing the PX-depleted stream and the use ofhydrogen, which requires an energy- and capital-intensive hydrogenrecycle loop.

Liquid Phase Isomerization

The EB-depleted stream 134 is fed to a xylene isomerization unit 150where the EB-depleted stream 134 is contacted with a xyleneisomerization catalyst under at least partially liquid phase conditionseffective to isomerize the PX-depleted, EB-depleted stream 134 backtowards an equilibrium concentration of the xylene isomers. Suitableconditions for the liquid phase isomerization include a temperature offrom about 200° C. to about 540° C., preferably from about 230° C. toabout 310° C., and more preferably from about 270° C. to about 300° C.,a pressure of from about 0 to 6895 kPa(g), preferably from about 1300kPa(g) to about 3500 kPa(g), a weight hourly space velocity (WHSV) offrom 0.5 to 100 hr⁻¹, preferably from 1 to 20 hr⁻¹, and more preferablyfrom 1 to 10 hr⁻¹. Generally, the conditions are selected so that atleast 50 wt % of the C₈ aromatics would be expected to be in the liquidphase.

Any catalyst capable of isomerizing xylenes in the liquid phase can beused in the xylene isomerization unit, but in one embodiment thecatalyst comprises an intermediate pore size zeolite having a ConstraintIndex between 1 and 12. Constraint Index and its method of determinationare described in U.S. Pat. No. 4,016,218, which is incorporated hereinby reference. Particular examples of suitable intermediate pore sizezeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-48,and MCM-22, with ZSM-5 and ZSM-11 being particularly preferred,specifically ZSM-5. It is preferred that the acidity of the zeolite,expressed as its alpha value, be greater than 300, such as greater than500, or greater than 1000. The alpha test is described in U.S. Pat. No.3,354,078; in the Journal of Catalysis, Vol. 4, p. 527 (1965); Vol. 6,p. 278 (1966); and Vol. 61, p. 395 (1980), each incorporated herein byreference as to that description. The experimental conditions of thetest used to determine the alpha values cited herein include a constanttemperature of 538° C. and a variable flow rate as described in detailin the Journal of Catalysis, Vol. 61, p. 395. A preferred catalyst isdescribed in U.S. Pat. No. 8,569,559, which is incorporated herein byreference.

The product of the liquid phase xylene isomerization process 150 is afirst isomerized stream 152 having a higher PX concentration than thePX-depleted, EB-depleted stream 134. The first isomerized stream 152 isthen recycled to the PX recovery unit 120 to recover additional PX andthe process is repeated to generate a so-called xylene isomerizationloop. To manage the level of EB in the liquid phase isomerization loopand prevent a buildup of EB, a purge stream 154 may be removed from thefirst isomerized stream 152 at regular intervals (which may bedetermined by one skilled in the art) and passed to the xyleneisomerization unit 140 where the EB may be converted.

Vapor Phase Isomerization

The EB-rich stream 132 is fed to a xylene isomerization unit 140 wherethe EB-rich stream 132 is contacted with a xylene isomerization catalystunder at least partially vapor phase conditions effective to isomerizethe PX-depleted, EB-rich stream 132 back towards an equilibriumconcentration of the xylene isomers. There are generally two types ofvapor phase isomerization catalysts—one that isomerizes the fourdifferent C8 aromatic compounds, including EB, to their equilibriumconcentrations and one that dealkylates EB to produce benzene andethylene and isomerizes the xylene isomers. Either catalyst may be usedfor the vapor phase isomerization unit 140.

EB Isomerization

In one embodiment, the EB-rich stream 132 is subjected to EBisomerization to produce a stream containing the C8 aromatic compoundsin equilibrium concentrations. The EB isomerization catalyst systemcomprises at least a first bed containing a xylene isomerizationcatalyst and a second bed downstream of the first bed and containing anethylbenzene isomerization catalyst. The beds can be in the same ordifferent reactors.

Typically, the xylene isomerization catalyst comprises an intermediatepore size molecular sieve having a Constraint Index within theapproximate range of 1 to 12, such as ZSM-5 (U.S. Pat. No. 3,702,886 andRe. 29,948); ZSM-11 (U.S. Pat. No. 3,709,979); ZSM-12 (U.S. Pat. No.3,832,449); ZSM-22 (U.S. Pat. No. 4,556,477); ZSM-23 (U.S. Pat. No.4,076,842); ZSM-35 (U.S. Pat. No. 4,016,245); ZSM-48 (U.S. Pat. No.4,397,827); ZSM-57 (U.S. Pat. No. 4,046,685); and ZSM-58 (U.S. Pat. No.4,417,780). Alternatively, the xylene isomerization catalyst maycomprise a molecular sieve selected from MCM-22 (described in U.S. Pat.No. 4,954,325); PSH-3 (described in U.S. Pat. No. 4,439,409); SSZ-25(described in U.S. Pat. No. 4,826,667); MCM-36 (described in U.S. Pat.No. 5,250,277); MCM-49 (described in U.S. Pat. No. 5,236,575); andMCM-56 (described in U.S. Pat. No. 5,362,697), with MCM-49 beingparticularly preferred. The molecular sieve may also comprise a EUOstructural type molecular sieve, with EU-1 being preferred, ormordenite. The entire contents of the above references are incorporatedby reference herein.

The xylene isomerization catalyst may also include ahydrogenation/dehydrogenation component, which may be the same materialpresent in the second, ethylbenzene isomerization catalyst, describedbelow. If the same hydrogenation/dehydrogenation component is used inboth catalysts, typically this component is present in a lower amount inthe xylene isomerization catalyst than in the ethylbenzene isomerizationcatalyst. More preferably, however, to reduce its ethylbenzeneconversion activity, the first catalyst composition does not contain ahydrogenation-dehydrogenation component.

In addition, it may be desirable to combine the molecular sieve of thexylene isomerization catalyst with another material resistant to thetemperature and other conditions of the process. Such matrix materialsinclude synthetic or naturally occurring substances as well as inorganicmaterials such as clay, silica, and/or metal oxides. The metal oxidesmay be naturally occurring or in the form of gelatinous precipitates orgels including mixtures of silica and metal oxides. Naturally occurringclays which can be composited with the molecular sieve include those ofthe montmorillonite and kaolin families, which families include thesubbentonites and the kaolins commonly known as Dixie, McNamee, Georgiaand Florida clays or others in which the main mineral constituent ishalloysite, kaolinite, dickite, nacrite or anauxite. Such clays can beused in the raw state as originally mined or initially subjected tocalcination, acid treatment or chemical modification.

In addition to the foregoing materials, the molecular sieve may becomposited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-berylia, silica-titania, as well as ternary compounds such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia,and silica-magnesia-zirconia. A mixture of these components could alsobe used. The matrix may be in the form of a cogel. The relativeproportions of molecular sieve component and inorganic oxide gel matrixon an anhydrous basis may vary widely with the molecular sieve contentranging from between about 1 to about 99 percent by weight and moreusually in the range of about 10 to about 80 percent by weight of thedry composite.

Typically the xylene isomerization catalyst typically has an alpha valueof about 4 to about 1000, such as from about 5 to about 80, with thepreferred value being inversely dependent on reactor temperature.

The EB isomerization catalyst composition is primarily intended toisomerize the ethylbenzene in the feed selectively to para-xylene, whileminimizing isomerization of the xylenes in the feed. The catalystcomposition typically comprises a molecular sieve having unidimensional10-membered ring pores. The phrase “unidimensional 10-membered ringpores” means that the pores of the molecular sieve are defined by10-membered rings of tetrahedrally coordinated atoms which extendessentially in one dimension so that the pores are substantially freefrom any intersecting pores. Examples of suitable molecular sieveshaving unidimensional 10-membered ring pores include ZSM-5, ZSM-12,SAPO-11, ZSM-23, ZSM-22, NU-87, ZSM-11, ZSM-50, ZSM-57, SAPO-41, ZSM-48,EU-1, and mordenite. Preferred molecular sieves are ZSM-5, ZSM-12 orEU-1.

The molecular sieve of the catalyst composition typically has an alphavalue of about 0.1 to about 20, for example from about 1 to about 5.

The molecular sieve used in the second catalyst composition isassociated with a hydrogenation/dehydrogenation component. Examples ofsuch components include the oxide, hydroxide, sulfide, or free metal(i.e., zerovalent) forms of Group VIII metals (i.e., Pt, Pd, Ir, Rh, Os,Ru, Ni, Co and Fe), Group VIB metals (i.e, Cr, Mo, W), Group IVA metals(i.e., Sn and Pb), Group VA metals (i.e., Sb and Bi), and Group VIIBmetals (i.e., Mn, Tc and Re). Combinations of catalytic forms of suchnoble or non-noble metals, such as combinations of Pt with Sn, may beused. The metal is preferably in a reduced valence state. The reducedvalence state of the metal may be attained, in situ, during the courseof the reaction, when a reducing agent, such as hydrogen, is included inthe feed to the reaction. Treatments such as coking or sulfiding mayalso be employed, especially at the start of a run with fresh catalyst,to modify the catalytic performance of the metal.

In one practical embodiment, the hydrogenation-dehydrogenation componentis a noble metal (i.e., Pt, Pd, Ir, Rh, Os and Ru) and particularly isplatinum. The amount of the hydrogenation-dehydrogenation component issuitably from about 0.001 to about 10 percent by weight, e.g., fromabout 0.03 to about 3 percent by weight, such as from about 0.2 to about1 percent by weight of the total catalyst although this will, of course,vary with the nature of the component, with less of the highly activenoble metals, particularly platinum, being required than of the lessactive base metals.

The EB isomerization catalyst composition may also include a binderand/or matrix material which may be the same as, or different from, anybinder and/or matrix material contained by the first catalystcomposition. In particular, the binder in the second catalystcomposition may be a zeolitic material such that the second catalystcomposition comprises a so-called “zeolite-bound zeolite” as describedin, for example, U.S. Pat. No. 6,517,807, the entire contents of whichare incorporated herein by reference. Thus, the second catalystcomposition may comprise a core zeolite having unidimensional10-membered ring pores, such as ZSM-48, bound with a high silica binderwhich is at least partly converted to a high silica zeolite (such asZSM-5 or ZSM-48) which at least partly covers the surface of the corezeolite. By ensuring that the zeolitic binder has a higher silica toalumina molar ratio than the core zeolite, the binder can lower thesurface activity of the core zeolite and hence reduce any unwantedxylene isomerization which would otherwise occur at the surface of thecore zeolite.

In general, the EB isomerization catalyst composition is different fromthe xylenes isomerization catalyst composition, for example bycontaining a different molecular sieve, having a lower alpha valueand/or by containing more or a more active hydrogenation/dehydrogenationcomponent.

The conditions employed in the xylene isomerization stage of the processof the second embodiment are not narrowly defined but generally includea temperature of from 250 to about 600° C., a pressure of from about 0to about 500 psig (100 to 3550 kPa), a weight hourly space velocity(WHSV) of between about 0.05 and about 50 hr-1, and a hydrogen, H2, tohydrocarbon, HC, molar ratio of between about 0.05 and about 20.

The conditions used in the ethylbenzene isomerization stage are also notnarrowly defined, but generally include a temperature of from about 250to about 600° C., a pressure of from about 0 to about 500 psig (100 to3550 kPa), a weight hourly space velocity (WHSV) of between about 0.01and about 20 hr-1, and a hydrogen, H2, to hydrocarbon, HC, molar ratioof between about 0.05 and about 20. Typically, the conditions include atemperature of from about 400 to about 500° C., a pressure of from about50 to about 400 psig (445 to 2870 kPa), a WHSV of between about 1 andabout 10 hr-1, and a H2 to HC molar ratio of between about 1 and about10.

In general, the xylene isomerization step and the ethylbenzeneisomerization step of the present process are carried out in fixed bedreaction zones containing the catalyst compositions described above. Thereaction zones may be in sequential beds in a single reactor, with theethylbenzene isomerization catalyst being located downstream of thexylene isomerization catalyst and with the feed being cascaded from thefirst to the second bed without intervening separation of light gases.As an alternative, the ethylbenzene isomerization catalyst and thexylene isomerization catalyst can be disposed in separate reactorswhich, if desired, can be operated at different process conditions, inparticular with the temperature of the ethylbenzene isomerizationreactor being higher than that of the xylene isomerization reactor.

The product of the vapor phase xylene isomerization process 140 is asecond isomerized stream 142 having a higher PX concentration than thePX-depleted, EB-rich stream 132. The second isomerized stream 142 isthen recycled to the PX recovery unit 120 to recover additional PX andthe process is repeated to generate a so-called xylene isomerizationloop. In embodiments, the second isomerized stream 142 is passed througha detoluenizing fractionation column 160 to produce a C⁷⁻ isomerizedstream 164 and a C₈₊ isomerized stream 162, which is passed through thexylenes re-run tower 110, before being recycled to the PX recovery unit120 to recover additional PX.

EB Dealkylation

In another embodiment, the EB-rich stream 132 is subjected to xylenesisomerization in which the EB in the stream can be dealkylated toproduce benzene. In this embodiment, where the ethylbenzene is removedby cracking/disproportionation, the para-xylene-depleted C8 stream isconveniently fed to a multi-bed reactor comprising at least a first bedcontaining an ethylbenzene conversion catalyst and a second beddownstream of the first bed and containing a xylene isomerizationcatalyst. The beds can be in the same or different reactors.

The ethylbenzene conversion catalyst typically comprises an intermediatepore size zeolite having a Constraint Index ranging from 1 to 12, asilica to alumina molar ratio of at least about 5, such as at leastabout 12, for example at least 20 and an alpha value of at least 5, suchas 75 to 5000. Constraint Index and its method of determination aredisclosed in U.S. Pat. No. 4,016, 218, which is herein incorporated byreference, whereas the alpha test is described in U.S. Pat. No.3,354,078 and in the Journal of Catalysis, Vol. 4, p. 527 (1965); Vol.6, p. 278 (1966); and Vol. 61, p. 395 (1980), each incorporated hereinby reference as to that description. The experimental conditions of thetest used herein include a constant temperature of 538° C. and avariable flow rate as described in detail in the Journal of Catalysis,Vol. 61, p. 395. Higher alpha values correspond with a more activecracking catalyst.

Examples of suitable intermediate pore size zeolites include ZSM-5 (U.S.Pat. Nos. 3,702,886 and Re. 29,948); ZSM-11 (U.S. Pat. No. 3,709,979);ZSM-12 (U.S. Pat. No. 3,832,449); ZSM-22 (U.S. Pat. No. 4,556,477);ZSM-23 (U.S. Pat. No. 4,076,842); ZSM-35 (U.S. Pat. No. 4,016,245);ZSM-48 (U.S. Pat. No. 4,397,827); ZSM-57 (U.S. Pat. No. 4,046,685);ZSM-58 (U.S. Pat. No. 4,417,780); EU-1; and mordenite. The entirecontents of the above references are incorporated by reference herein.Preferred zeolites are ZSM-5, ZSM-12 or EU-1.

The zeolite employed in ethylbenzene conversion catalyst typically has acrystal size of at least 0.2 microns and exhibits an equilibriumsorption capacity for xylene, which can be either para, meta, ortho or amixture thereof, of at least 1 gram per 100 grams of zeolite measured at120° C. and a xylene pressure of 4.5+0.8 mm of mercury and anortho-xylene sorption time for 30 percent of its equilibriumortho-xylene sorption capacity of greater than 1200 minutes (at the sameconditions of temperature and pressure). The sorption measurements maybe carried out gravimetrically in a thermal balance. The sorption testis described in U.S. Pat. Nos. 4,117,026; 4,159,282; 5,173,461; and Re.31,782, each of which is incorporated by reference herein.

The zeolite used in the ethylbenzene conversion catalyst may beself-bound (no binder) or may be composited with an inorganic oxidebinder, with the zeolite content ranging from between about 1 to about99 percent by weight and more usually in the range of about 10 to about80 percent by weight of the dry composite, e.g., about 65% zeolite withabout 35% binder. Where a binder is used, it is preferably non-acidic,such as silica. Procedures for preparing silica bound ZSM-5 aredescribed in U.S. Pat. Nos. 4,582,815; 5,053,374; and 5,182,242,incorporated by reference herein.

In addition, the ethylbenzene conversion catalyst typically comprisesfrom about 0.001 to about 10 percent by weight, e.g., from about 0.05 toabout 5 percent by weight, e.g., from about 0.1 to about 2 percent byweight of a hydrogenation/dehydrogenation component. Examples of suchcomponents include the oxide, hydroxide, sulfide, or free metal (i.e.,zero valent) forms of Group VIIIA metals (i.e., Pt, Pd, Ir, Rh, Os, Ru,Ni, Co, and Fe), Group VIIA metals (i.e., Mn, Tc, and Re), Group VIAmetals (i.e., Cr, Mo, and W), Group VB metals (i.e., Sb and Bi), GroupIVB metals (i.e., Sn and Pb), Group IIIB metals (i.e., Ga and In), andGroup IB metals (i.e., Cu, Ag and Au). Noble metals (i.e., Pt, Pd, Ir,Rh, Os and Ru) are preferred hydrogenation/dehydrogenation components.Combinations of catalytic forms of such noble or non-noble metal, suchas combinations of Pt with Sn, may be used. The metal may be in areduced valence state, e.g., when this component is in the form of anoxide or hydroxide. The reduced valence state of this metal may beattained, in situ, during the course of a reaction, when a reducingagent, such as hydrogen, is included in the feed to the reaction.

The xylene isomerization catalyst employed in this embodiment typicallycomprises an intermediate pore size zeolite, e.g., one having aConstraint Index between 1 and 12, specifically ZSM-5. The acidity ofthe ZSM-5 of this catalyst, expressed as the alpha value, is generallyless than about 150, such as less than about 100, for example from about5 to about 25. Such reduced alpha values can be obtained by steaming.The zeolite typically has a crystal size less than 0.2 micron and anortho-xylene sorption time such that it requires less than 50 minutes tosorb ortho-xylene in an amount equal to 30% of its equilibrium sorptioncapacity for ortho-xylene at 120° C. and a xylene pressure of 4.5+0.8 mmof mercury. The xylene isomerization catalyst may be self-bound form (nobinder) or may be composited with an inorganic oxide binder, such asalumina. In addition, the xylene isomerization catalyst may contain thesame hydrogenation/dehydrogenation component as the ethylbenzeneconversion catalyst.

Using the catalyst system described above, ethylbenzenecracking/disproportionation and xylene isomerization are typicallyeffected at conditions including a temperature of from about 400° F. toabout 1,000° F. (204 to 538° C.), a pressure of from about 0 to about1,000 psig (100 to 7000 kPa), a weight hourly space velocity (WHSV) ofbetween about 0.1 and about 200 hr-1, and a hydrogen, H2 to hydrocarbon,HC, molar ratio of between about 0.1 and about 10. Alternatively, theconversion conditions may include a temperature of from about 650° F.and about 900° F. (343 to 482° C.), a pressure from about 50 and about400 psig (446 to 2859 kPa), a WHSV of between about 3 and about 50 hr-1and a H2 to HC molar ratio of between about 0.5 and about 5. The WHSV isbased on the weight of catalyst composition, i.e., the total weight ofactive catalyst plus, if used, binder therefor.

The product of the vapor phase xylene isomerization process 140 is asecond isomerized stream 142 having a higher PX concentration than thePX-depleted, EB-rich stream 132 and containing benzene. The secondisomerized stream 142 is preferably processed through a detoluenizingfractionation 160 to produce a C⁷⁻ isomerized stream 164 and a C₈₊isomerized stream 162, which is then recycled to the PX recovery unit120 to recover additional PX.

Conducting xylenes isomerization under liquid phase conditions producesless C₉₊ aromatics than xylenes isomerization under vapor phaseconditions. Therefore, the first isomerized stream 152 may be providedto the xylenes fractionation tower 110 at a higher tray location thanthe second isomerized stream 142 or the C₈₊ isomerized stream 162,yielding greater energy savings. . Furthermore, a significant portion ofthe first isomerized stream 152 may bypass the xylenes fractionationtower 110 and directly enter the PX recovery unit 120, thereby savingenergy by avoiding the re-fractionation altogether.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations andmodification is not necessarily illustrated herein without departingfrom the spirit and scope of the invention.

Trade names used herein are indicated by a ™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated.

The invention claimed is:
 1. An apparatus for the production ofpara-xylene comprising: a para-xylene recovery unit, which produces apara-xylene-rich stream and at least one para-xylene-depleted streamfrom a hydrocarbon feed, fluidly connected to a divided wall column inwhich the at least one para-xylene-depleted stream is separated into anethylbenzene-rich stream, which contains a majority portion of theethylbenzene from the at least one para-xylene-depleted stream, and anethylbenzene-depleted stream, which contains a minor portion of theethylbenzene from the at least one para-xylene-depleted stream; a liquidphase isomerization unit fluidly connected to the divided wall column toisomerize the ethylbenzene-depleted stream back towards an equilibriumconcentration of the xylenes isomers and produce a first isomerizedstream having a higher para-xylene concentration than thepara-xylene-depleted stream; a vapor phase isomerization unit fluidlyconnected to the divided wall column to isomerize the ethylbenzene-richstream back towards an equilibrium concentration of the xylenes isomersand produce a second isomerized stream having a higher para-xyleneconcentration than the para-xylene-depleted stream; and a xylenesfractionation column fluidly connected to the liquid phase isomerizationunit, the vapor phase isomerization unit and the para-xylene recoveryunit, wherein the xylenes fractionation column is downstream of theliquid phase isomerization unit and the vapor phase isomerization unitand upstream of the para-xylene recovery unit.
 2. The apparatus of claim1, further comprising a detoluenizing fractionation column fluidlyconnected to the vapor phase isomerization unit and the xylenesfractionation column, wherein the detoluenizing fractionation column isdownstream of the vapor phase isomerization unit and upstream of thexylenes fractionation column.
 3. The apparatus of claim 1, wherein thefirst isomerized stream is sent to a higher location in the xylenesfractionation column than the second isomerized stream.
 4. The apparatusof claim 1, wherein the para-xylene recovery unit produces twopara-xylene-depleted streams.